The coaxial cables commonly used today for the transmission of RF signals, such as cable television signals and cellular telephone broadcast signals, include a core containing an inner conductor, a metallic sheath surrounding the core and serving as an outer conductor, and in some instances a protective jacket that surrounds the metallic sheath. A dielectric surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. In many known coaxial cable constructions, an expanded foam dielectric surrounds the inner conductor and fills the space between the inner conductor and the surrounding metallic sheath.
The design of coaxial cables has traditionally been a balance between the electrical properties (e.g., high signal propagation, and low attenuation) and the mechanical or bending properties of the cable. For example, in some coaxial cable constructions, air and plastic spacers are used between the inner conductor and the outer conductor to reduce attenuation and increase signal propagation of the cable. Nevertheless, the plastic spacers that are placed between the inner and outer conductors do not adequately support the outer conductor in bending. Thus, the outer conductor is subject to buckling, flattening or collapsing during bending which can render the cable unusable. One alternative has been to use a foam dielectric between the inner and outer conductors as described above. However, although the bending properties are improved, the rate at which the signals are propagated is typically reduced.
One recent advance in the coaxial cable industry for RF cables has been the construction of larger diameter cables. Larger diameter cables generally possess a greater average power rating and reduced attenuation over relatively smaller diameter cables. Unfortunately, however, because these cables have large diameters, they are typically not as flexible as their smaller diameter counterparts. As a result, there is a greater level of difficulty in installing these cables.
Another problem with larger diameter cables and cables generally is that moisture in the cable can corrode the conductors thus negatively affecting the electrical and mechanical properties of the cable. In particular, during installation of the cable, moisture can enter the cable at the connectors. This moisture can also travel within the cable through the foam or air dielectric or along interfaces in the cable, e.g., between a foam dielectric and a metallic sheath.
Several methods have been proposed to prevent moisture from entering the cable and being transported through the cable. For example, hydrophobic, adhesive compositions have been applied at interfaces in the cable to prevent moisture from moving along these interfaces. Water-blocking compositions have also been used at other locations in the cable to limit water transport in the cable. In addition, hydrophilic, moisture-absorbent materials have been used in cables to act as water-blocking materials. These hydrophilic materials not only water-block the cable but also remove moisture that is present in the cable.
Copending U.S. patent application Ser. No. 08/911,538 to Moe et al., filed on Aug. 14, 1997, which has been incorporated herein in its entirety by reference, proposes a new cable construction using an inner conductive tube thus reducing the cost of the cable and providing a cable with good flexibility. Although this new construction provides numerous benefits, using an inner conductive tube creates the possibility of moisture moving through the cable. In particular, moisture can enter the tube during connectorization especially in moist conditions. This moisture can be transported within the inner conductive tube thereby causing corrosion of the inner conductive tube. Therefore, there is a need in the art to avoid moisture transmission in this region of the cable and the attendant potential effects upon the electrical and mechanical properties of the cable.